Fatigue Strength and Endurance Limit
It is necessary to determine the fatigue strength of a material by experiment. This is achieved by subjecting test specimens to repeated loads of specified magnitudes and determining the number of cycles required to produce failure. Such results are plotted graphically with the alternating stress as the ordinate and the number of cycles to produce failure as the abscissa; it is usual to make these plots on logarithmic scales. The general shape of the curves so obtained is indicated in Fig. 3. 2, from which it can be seen that the higher the stress level the fewer the number of cycles required producing failure. For ferrous materials [Fig. 3. 2(a)], there is a tendency for the curve to become asymptotic, and the stress level at which this occurs is termed the endurance limit (for steels this corresponds to a life of approximately 106) . Non-ferrous materials do not possess this characteristic (or if they do, it is certainly at an extremely high number of cycles), although there is a gradual leveling out of the curve [Fig. 3. 2(b)]. However, a non-ferrous material is frequently referred to as having an endurance limit, and this usually means the fatigue strength corresponding to 108 or 5 x 108 ; it is more meaningful to use the term fatigue strength when referring to non-ferrous, and to qualify this by stating the number of cycles to which it refers.
Many different types of machines have been developed for the laboratory testing of specimens under different conditions of loading, and these are discussed by Marin and Fanner. The most usual test method is that known as rotating bending, in which a bending moment is applied to a test specimen whilst the specimen itself rotates; thus, the outer fibers of the specimen are subjected to stresses which fluctuate between equal values in tension and compression for each revolution. When applying data obtained by test to a design problem, the method by which the fatigue data have been obtained must be carefully considered. For example, the fatigue strength obtained by direct tension compression tests, in general, gives low fatigue strengths than obtained in rotating bending, and the fatigue strength under conditions of reversed torsion is different again. Breitling Replica Watches
Processing your request, Please wait....
